ox THE OCCUEliEXCE OF SHEATH CELLS AND THE 

 XATUEE OF THE AXO^^E SHEATHS IX THE CEXTBAL 

 XEEVOUS SYSTEM. 



IRVING HARDESTY. 

 {From the Hearst Anatomical Laboratory of the University of California.) 



In a previous paper dealing with the development of the neuroglia 

 tissue, a brief note was made of the occasional observance of certain half- 

 moon or seal-ring cells encircling the medullated nerve fibers of the devel- 

 oping spinal cord of the foetal pig. ^ It was stated that these cells appeared 

 more numerous in the spinal cords of pigs between 16 and 25 centimeters, 

 the period of most active medullation, and that in transverse sections they 

 appear as seal-rings or crescents encircling the medullating axones and 

 that they closely resemble the " nerve corpuscles " or " Schwann's corpus- 

 cles " which have been described clasping the medullated fibers of the 

 peripheral nervous system. It was also suggested that, while in the light 

 of recent investigations these cells have probably little or nothing to do 

 with the formation of the myelin of the medullary sheath, they may 

 have to do with the development of the supporting framework of that 

 sheath. 



Adamkiewicz, who first described the cells upon the fibers of the devel- 

 oping peripheral nerves, referred to them as " nerve-corpuscles," and -by 

 way of distinction I have referred to the similar cells observed upon the 

 fibers of the central system as " seal-ring cells." This name only applies 

 to their appearance in transverse section and is non-commital as to their 

 particular function. 



The purpose of this paper is to give a further description of these seal- 

 ring cells based upon a more extended study of their occurrence and varia- 

 tions, and to offer a suggestion as to their relation to the medullary sheath. 

 Some attention has necessarily been given to the nature of the support- 

 ing framework of the medullar}- sheath. 



Material axd Methods. 



Pig material has been used almost exclusively in the observations in 

 that the different ages could be easily obtained. All the study has been 



American- Journal of Axatomy. — Vol. IV. 

 26 



